    package com.jayden.ajax;

    import java.io.IOException;
    import java.io.Serializable;

    import javax.servlet.ServletException;
    import javax.servlet.annotation.WebServlet;
    import javax.servlet.http.HttpServlet;
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;

    @WebServlet("/XHRServlet")
    public class XHRServlet extends HttpServlet implements Serializable {
        // 序列化版本号，作用是在反序列化时检查类的版本是否一致
        private static final long serialVersionUID = 1L;
        // 已有的用户名数组
        private String[] users = {"张三", "李四", "王五"};

        protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            response.setContentType("text/html;charset=utf-8");
            // 新闻标题数组对应颜色
            String[] news = {"新闻1", "新闻2", "新闻3"};
            String[] colors = {"pink", "lightgreen", "orange"};
            // 随机生成一个新闻标题   
            int index = (int) (Math.random() * news.length);
            String title = news[index];
            String color = colors[index];
            // 返回新闻标题   
            response.getWriter().write("<h3 style=\"color=" + color + "\">" + title + "</h3>");
        }

        protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            
            // 设置请求编码格式
            request.setCharacterEncoding("UTF-8");
            // 设置响应编码格式
            response.setContentType("text/html;charset=UTF-8");
            // 获取请求参数
            String todo = request.getParameter("todo");
            System.out.println("todo: " + todo);
            // 如果 todo 为 check，进行冲突检测
            if ("check".equals(todo)) {
                // 获取用户名
                String username = request.getParameter("username");
                System.out.println("用户填入的 username: " + username);
                // 判断用户名是否已存在
                boolean flag = false;
                for (String user : users) {
                    if (user.equals(username)) {
                        flag = true;
                        break;
                    }
                }
                // 返回是否可用的提示
                if (flag) {
                    System.out.println(username + " 已存在");
                    response.getWriter().write("用户名已存在");
                } else {
                    System.out.println(username + " 可用");
                    response.getWriter().write("用户名可用");
                }
            } else if ("register".equals(todo)) {
                // 如果 todo 为 register，进行注册
                response.getWriter().write("注册成功");
                System.out.println("用户注册新账号：" + request.getParameter("username"));
            }
        }
    }